1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same, and is preferably applied to a broadcasting TV camera, a video camera, a digital still camera, a silver salt film camera, or the like.
2. Description of the Related Art
Recently, for an image pickup apparatus such as a TV camera, a silver salt film camera, a digital still camera, a video camera, or the like, a zoom lens having a high zoom ratio and a high optical performance is desired. As a zoom lens with a high zoom ratio, a four-group zoom lens of a positive lead type and a telephoto type in which a lens unit having a positive refractive power is arranged at the most object side and is constituted by four lens units as a whole is known. For example, a four-unit zoom lens which is constituted by a first lens unit having a positive refractive power for a focus operation, a second lens unit having a negative refractive power for a magnification varying operation, a third lens unit having a positive or negative refractive power for correcting an image plane displacement, and a fourth lens unit having a positive refractive power for forming an image is known. In such four-group zoom lenses, a four-group zoom lens having a high optical performance, which includes an optical material having an abnormal dispersibility is used to well correct a chromatic aberration is known (Japanese Patent Laid-open No. 2006-145823 and Japanese Patent Laid-open No. 2004-109993).
The zoom ratio of the four-group zoom lens of the positive lead type having the configuration described above is comparatively easily heightened. In order to obtain a high optical performance in the four-group zoom lens, it is important to well correct a chromatic aberration of magnification at a wide-angle end and an on-axis chromatic aberration at a telephoto end. If an optical material having an abnormal dispersibility is used, it is easy to well correct the chromatic aberration of magnification or the on-axis chromatic aberration. However, if a region (a lens) which appropriately absorbs the influence on other aberrations is not provided in this case, the correction balance of the chromatic aberration is deteriorated.
Commonly, at the image plane side with reference to a position of a stop, the height of incident light of an on-axis light beam on a lens surface is the maximum at the wide-angle end. Therefore, if a lens constituted by an optical material having an abnormal dispersibility is used at the image plane side with reference to the position of the stop, the influence is given to the on-axis chromatic aberration as well as the chromatic aberration of magnification. Therefore, it is difficult to correct both of them in a balanced manner. Accordingly, in a zoom lens which is bright and has a high zoom ratio, it is important to ensure a correction effect of the chromatic aberration of magnification and appropriately corrects the on-axis chromatic aberration to correct them in a balanced manner. Particularly, in the four-group zoom lens of the positive lead type, an appropriate setting of the lens configuration of the fourth lens unit which does not move for a zoom operation is an important factor in order to obtain a high optical performance over the entire zoom range. If the lens configuration of the fourth lens unit is inappropriate, various kinds of aberrations including the chromatic aberration are increased, and therefore it is difficult to obtain the high optical performance throughout the entire zoom range.